In the construction and operation of large combustion installations, for example, steam-generating plants for steam turbines (e.g. in coal-fired electricity-generating power plants), increased attention is required to avoid the emission of toxic and noxious components into the atmosphere. In particular, the release of sulfur and sulfur compounds must be held as low as possible. On the other hand, for economical reasons, it is desirable to utilize in the combustion process so called low grade coal, e.g. brown coal or low-rank bituminous coal, a fuel which, upon combustion, generates large quantities of such pollutants.
The most significant pollutants are sulfur dioxide and sulfur trioxide and these, at least, must be removed to the greatest possible extent from the flue gases. It should be self-understood that the removal of sulfur and sulfur compounds requires flue gas cleaning apparatus which is expensive to install and to operate.
To reduce the emission of sulfur and sulfur compounds into the flue gas in a gaseous form, it has already been proposed to bind the sulfur from the fuel in a solid ash or particulate product. To this end, it has been proposed to introduce additives into the combustion chamber so that they will absorb, e.g. by chemical reaction, at least part of the sulfur which is liberated by the fuel combustion within this chamber.
Chemical binding agents for sulfur in this manner have hitherto included finely divided calcium oxide or calcium carbonate which were blown into the combustion chamber.
In coal-fired combustion chambers, moreover, in which the coal is subjected to a mill-drying circulation, the calcium oxide or calcium carbonate can be added to the coal before the coal enters the mill so that the milling operation blends the sulfur-binding additive with the coal.
The two components, namely the coal and the calcium compound are then introduced together through the coal-dust burner into the fire box or combustion chamber.
This dry additive technique, while successful in reducing the amount of gaseous sulfur which must be scrubbed or otherwise removed from the flue gas subsequently, does not fully satisfy all desired requirements at least in part because the type of coal used plays a significant role in the efficiency of the process. For example, it has been found that many brown coal types cannot be adequately desulfurized by the dry additive process, possibly because high combustion chamber temperatures are developed which may result in a dead burning of the calcium oxide.
It has also been suggested to introduce a coal gas to the combustion chamber, e.g. a recycled flue gas, so that its available heat state can be utilized to maintain optimum temperature conditions for the reaction of the additive with the sulfur in the flue gas. An apparatus utilizing this process is described in greater detail below.
To improve the binding of the sulfur in the additive, it has been suggested to raise the relative humidity of the flue gas before it enters the dust filter by cooling it to a lower temperature. The cooling can be effected via a heat exchanger and/or by the spraying of water into the flue gas.
The additive which has not reacted in the combustion chamber can then react in the filter unit with any remaining sulfur in the flue gas or with sulfur compounds which may remain in the flue gas to remove them.
In this connection it has been found that a corresponding cooling of the flue gas to the dewpoint and therebelow is not possible by a conventional contact cooler or by a water spray technique. In a contact cooler there is a formation of deposits upon the heat exchange surfaces which is detrimental to the process and, with a water injection process, nonevaporated water droplets collect upon the walls of the flue gas passages and apparatus. The result is a number of corrosion problems and a tendency for crusts to bake onto the surfaces, thereby leading to interruption of operations, downtime for repair and expensive maintenance procedures.
Tests with cloth filters and especially bag and tube filters for recovering the dust have shown that effective binding of the sulfur dioxide (90% and greater) can only be obtained when the temperature of the filter layer is in the region of the water dewpoint of the flue gas. Most advantageously, the "overheating" of the flue gas at the filter layer should be no greater than 5.degree. C., i.e. the temperature of the flue gas should not exceed the water dewpoint at the filter by more than 5.degree. C.